Constant speed control of a spindle motor is always pursued by industry. A typical application of such a speed control is hard disk drive (HDD) spindle motors. Now HDD spindle motors employ a zero-current-window (ZCW) method to detect the transient moments of zero-crossing points (ZCP) of each phase back electromotive force (BEMF) voltage. The rotor position is detected and speed can be calculated to realize a close-loop control with a speed jitter around ±0.1˜0.2%.